1. Field of the Invention
The present invention relates generally to an electrostatic capacitance type alcohol concentration measuring system applicable to a fuel injection system injecting a mixture fuel of gasoline and alcohol (methanol) into an internal combustion engine.
2. Description of the Background Art
When a mixture of gasoline and alcohol (methanol) is used in place of pure gasoline as the fuel of the engine, the stoichiometric air/fuel mixture ratio is naturally changed and it is necessary to change the amount of fuel injected along with the ignition timing and the like.
When gasoline containing 0% of alcohol is supplied to the engine, an injection pulsewidth Ti to be supplied to a corresponding fuel injection valve, the fuel injection pulsewidth corresponding to the fuel injection amount, is determined on the basis of the following equation: EQU Ti=Tp.times..alpha..times..alpha.'.times.C.sub.oef +Ts (1)
wherein,
Tp denotes a basic fuel injection quantity;
.alpha. denotes a correction coefficient for air/fuel mixture ratio feedback control;
.alpha.' denotes a basic air-fuel mixture ratio adaptive correction coefficient;
C.sub.oef denotes various correction coefficients; and
Ts denotes a battery voltage variation coefficient.
It is noted that the correction factor (coefficient) .alpha. is corrected on the basis of an oxygen concentration signal derived from an oxygen (O.sub.2) sensor disposed in an exhaust pipe of the engine and the learning correction coefficient .alpha.' is corrected through a learning method on the basis of the basic fuel injection quantity Tp and engine revolution speed N.
Consequently, the stoichiometric air/fuel mixture ratio is controlled to achieve the value of 14.7.
In the way described above, although the stoichiometric air/fuel mixture ratio for pure gasoline is 14.7, the air/fuel mixture ratio A/F is controlled to provide a value of 6.5 in a case of the fuel having the ratio of 100% alcohol concentration of methanol. Therefore, it is necessary to change the stoichiometric air/fuel mixture ratio by about twice in the range of the alcohol concentration of 0 through 100%.
Hence, in a case where the alcohol blended gasoline is used as the fuel, it is necessary to calculate the fuel injection quantity Ti' from the equation (1) as follows: EQU Ti'=M.sub.k .times.Tp.times..alpha..times..alpha.'.times.C.sub.oef +Ts(2)
wherein, M.sub.k denotes a constant determined according to the alcohol concentration.
Therefore, in a case where the alcohol blended gasoline is used for the fuel supplied to the engine, a measuring instrument such as an alcohol sensor is installed to generate an output voltage corresponding to the alcohol concentration and to calculate the equation of (2) on the basis of the generated output voltage value.
The measuring instrument described above includes a resistance type alcohol concentration measuring system which detects the concentration of alcohol from electric conductivities of the gasoline and alcohol; an electrostatic capacitance type alcohol concentration measuring apparatus utilizing a change in a dielectric constant (permitivity) of the alcohol blended gasoline; and an optical type alcohol concentration measuring apparatus utilizing a light index of refraction.
FIGS. 7 through 13 show a fuel injection control system disclosed in U.S. Pat. Nos. 5,205,151 and 5,060,619 which incorporates the alcohol sensor of the type which utilizes the change in the electrostatic capacitance described above.
The arrangement of the whole engine shown in FIG. 7 includes an internal combustion engine 1, a fuel injector 2, an induction manifold 3, an air cleaner 4, an air flow meter 5, an exhaust manifold 6 which includes an O.sub.2 sensor (not shown), a fuel tank 7 containing an alcohol blended gasoline 8, a fuel pump 9, a fuel supply conduit 10, a filter 11 which is disposed in the supply conduit 10, a pressure regulator 12 and a return conduit via which fuel is returned to the fuel tank 7.
The arrangement further includes an alcohol concentration sensor 14 which is disposed in the fuel supply conduit at a location downstream of the filter 11 and which is arranged to output a signal indicative of the amount of alcohol contained in the fuel being pumped through the supply conduit 10.
This alcohol concentration sensor 14 is constituted by a pair of electrode plates in a form of a pair of flat parallel metallic plates or of a coaxial cylindrical tube installed in the fuel conduit 10.
In the case of the pair of flat parallel plates described above, an electrostatic capacitance Cs is expressed as follows: EQU Cs=.epsilon.S/d (3)
wherein .epsilon. denotes an permitivity, S denotes an electrode area, and d denotes a distance in space between electrodes.
An electrostatic capacitance defined as above is detected by an electrostatic capacitance detector 15.
An oscillation frequency f of an LC oscillator 16 is expressed by the following equation (4): ##EQU1##
wherein L denotes an inductance; and
Co denotes a capacitance that the circuit inherently has.
In addition, a frequency/voltage (F/V) converter 17 to convert the oscillation frequency f into the detection voltage V is provided. Furthermore, an inverting amplifier 18 inverts and amplifies the detected voltage V from the F/V converter 17 and outputs an output voltage Vo.
In more detail, the alcohol blended gasoline 8 has the relationship of the alcohol concentration M to the permitivity .epsilon. shown by the characteristic of FIG. 9.
FIG. 10 shows a relationship between the electrostatic capacitance Cs detected by the electrostatic capacitance detector 15 and alcohol concentration M.
The detected voltage V produced by means of the F/V converter 17 is represented by the characteristic shown in FIG. 11.
This is inverted and amplified with the inverting amplifier 18. Consequently, the output voltage Vo has the characteristic shown in FIG. 12.
Thus, the alcohol concentration measuring apparatus 14 can provide the output voltage Vo having the characteristic shown in FIG. 12 with respect to the alcohol concentration M.
Numeral 19 denotes a temperature responsive sensor comprising a thermistor or posister detecting a fuel temperature (hereinafter, referred to as a fuel temperature) of the alcohol blended gasoline 8, installed mid-way in the fuel conduit 10. The detected temperature t by means of the temperature sensor 19 is input into an alcohol concentration temperature correcting apparatus 21 as will be described later (and a fuel injection pulsewidth calculation circuit 22), the alcohol concentration M being corrected in terms of the temperature.
In addition to the above-described data inputs, the control unit receives a plurality of other data such as an input data on engine crank angle sensor 23, an engine coolant temperature and so on.
As shown in FIG. 9, the capacitance type alcohol concentration sensor 14 is such that as the alcohol concentration M increases, its permitivity .epsilon. (dielectric constant) accordingly increases. Therefore, the change in the permitivity causes the change in the electrostatic capacitance Cs, the change in the capacitance being detected and being monitored from the output voltage change.
It is noted that the permitivity .epsilon. varies not with the change in the alcohol concentration but only also with the change in temperature. Consequently, the output voltage Vo from the electrostatic capacitance type alcohol concentration measuring apparatus 14 has a temperature dependent characteristic with respect to a fuel temperature t as shown in FIG. 13.
That is to say, the output voltage Vo of the alcohol concentration measuring apparatus M generated is such that the output voltage increases as the fuel temperature decreases with respect to the same alcohol concentration M.
Therefore, the control unit 20 performs the function of the alcohol concentration temperature correction apparatus 21 by software. An input of the alcohol concentration temperature correction apparatus 21 is connected to the alcohol concentration measuring apparatus 14 and the temperature responsive sensor 19 and an output thereof is connected to the fuel injection quantity calculating apparatus 22. The alcohol concentration temperature correcting apparatus 21 corrects the output voltage Vo from the inverting amplifier 18 in terms of the temperature on the basis of the detected temperature t from the temperature responsive sensor 19. The alcohol concentration temperature correcting apparatus 21 internally includes storage elements such as RAM and ROM in which is stored a temperature correction map 21A. The map 21A represent a relationship between the alcohol concentration M and output voltage Vo for each detected temperature t, so that, for example, a standard output voltage after correction corresponding to 20.degree. C. and a standard alcohol concentration after correction is output.
Furthermore, the fuel injection calculation apparatus 22 in the control unit 20 has an input connected to the alcohol concentration temperature correction apparatus 21, crank angle sensor 23, air-flow meter 5, an oxygen sensor and a coolant temperature sensor (not shown) and an output connected to a fuel injection valve 2 through which the engine fuel is injected.
The fuel injection quantity calculation apparatus 22 described above calculates the basic fuel injection quantity Tp on the basis of the engine revolution speed N from the crank angle sensor 23 and the intake air quantity Q from the air-flow meter 5, calculates the fuel injection quantity Ti' according to the equation (2) on the basis of the standard output voltage after correction or standard alcohol concentration after correction from the alcohol concentration temperature correction apparatus 21 and other signals from the various sensors, and outputs a fuel injection pulse having a duty ratio corresponding to the calculated fuel injection quantity Ti' to the fuel injection valve 2.
When the alcohol concentration measuring apparatus 14 is applied to the fuel injection controlling apparatus, the structure thereof is as described above. The electrostatic capacitance detector 15 detects the electrostatic capacitance Cs from the equation (3) corresponding to the alcohol concentration M. The oscillation circuit 16 oscillates at the frequency f according to the equation (4) corresponding to the alcohol concentration M, outputs the detected voltage V corresponding to the the frequency f by means of the f-V converter 17, and outputs the output voltage Vo after inverted amplification in the inverting amplifier 18.
On the other hand, the control unit 20 includes the alcohol concentration temperature correction apparatus 21 which serves to carry out the temperature correction of the alcohol concentration on the basis of the output voltage Vo from the alcohol concentration measuring apparatus 14 and outputs to the fuel injection quantity calculating apparatus 22 the standard output voltage or standard alcohol concentration corresponding to, e.g., 20.degree. C.
Thus, the calculation of the fuel injection quantity Ti' according to the equation (2) on the basis of the standard alcohol concentration without influence of the temperature can be executed so that highly accurate injection of fuel can be achieved.
However, in the previously proposed alcohol concentration measuring apparatus described above, since a low oscillation frequency (100 KHz or less) is used as the oscillation frequency f of the LC type oscillator 16, its oscillation frequency has variations in frequency due to the influence of conductive materials (metallic ions etc.,.) impregnated in the fuel so that the detected voltage becomes unstable. Therefore, the calculation of the fuel injection quantity Ti' by the fuel injection quantity calculating apparatus 22 becomes inaccurate and accurate control of the air/fuel mixture ratio cannot be achieved.